The guys at the shop I work at just got done making a custom sub enclosure that fits under the seat of a 4 door JK. It will house (2) 8" or 10" down-firing subs.
A national sub box manufacturer contacted us to make the first prototype so they can start producing them. We designed and fabricated the 8" enclosure for one of our customers (see pics) and will be making the 10" box for actual production.

Rather than spending the money and hassle of this type of box, Kicker makes a 8" loaded box that fits PERFECT under the back seat. I have one under mine and it does as good of a job as an 8" can in that configuration.

If I put another one under there, it would be pretty nice and the cost is pretty good.

I am working on an under seat box to house 2 10 inch subs.
Got a little time to throw together the first mock up of the sub box.
Inner structure..
[IMG][/IMG]

The fiberglass bottom.

Front..

The rear. a few gaps to be taken care of in the final version.

After accounting for 4 oz Coremat and a layer of epoxy/quartz to deaden any resonance from transferring to the metal, and the speakers displacement, I have .77 feet per side. With a little more angle on the front and rounding the sides out from under the seat, as well as building in tighter to the hinges I could squeeze out 1 foot of airspace.